Mapper

NES games come in cartridges, and inside of those cartridges are various circuits and hardware. Different games use different circuits and hardware, and the configuration and capabilities of such cartridges is commonly called their mapper. Mappers are designed to extend the system and bypass its limitations, such as by adding RAM to the cartridge or even extra sound channels. More commonly though, mappers are designed to allow games larger than 40K to be made.

The term "mapper" arises from the concept of memory mapping: translating memory hardware into the CPU's and PPU's address spaces. A memory map describes which addresses correspond to which physical locations of memory.

Contents

Common capabilities

Multiple functions can be performed by the hardware and circuitry on a cartridge's printed circuit board:

Mapping of the CHR and PRG chips into the NES's address space, which may be expanded by…

(often) Bank switching: dynamically changing the memory mapping of cartridge memory into the NES's address spaces. By doing so, the cartridge may have more data than the address space would allow, allowing larger games to be made.

(often) Using a battery-backed volatile RAM chip, or rarely an EEPROM, to maintain save data between play sessions

(always) Nametable Mirroring: controlling which way the nametables are arranged, as the NES only has two nametables' worth of CIRAM but four tables addressed.

(rarely) Providing additional sound-generation. The Famicom sent its audio through the cartridge port to allow the cartridge to mix in additional channels; the NES does not without modification.

(rarely) Altering how the attribute table is mapped to provide finer attribute control.

(rarely) Other sundry functions like fast multiplication.

Most mappers fall into one of two categories: discrete logic, and ASIC based. Some discrete logic mappers are susceptible to bus conflicts. Nintendo uses the term Memory Management Controller (or MMC for short) for its ASIC mappers ("Why Game Paks Never Forget" article in Nintendo Power) (note: it may have originally stood for "Multi Memory Controller", at least according to Japanese).

Naming scheme

Discrete logic mappers are often referred to by the name of a board that they are commonly used in (e.g. "UNROM").
ASIC mappers are named after the ASIC (e.g. "MMC1" or "FME-7"), except in boards that use an ASIC in an unusual way (such as "TQROM", "TLSROM", or "NES-EVENT").
The emulation community generally refers to mappers by a numbering scheme that originated with the iNES emulator (e.g. "mapper 002").

The notation used in Disch's docs describing mappers is explained here.

To allocate a mapper number, you should have A. a hardware implementation or B. an emulator implementation and a sketch of hardware. You should also have a publicly-available dump of an existing commercial cart or most of a homebrew game written.

iNES 1.0 mapper grid

This is the plane 0 table. These mappers do not require a NES 2.0 header.

Most icons next to mapper numbers refer to publishers.
Nintendo-made boards with numerous publishers get the Nintendo icon; Nintendo-made boards dominated by one publisher get that publisher's icon.
Other icons refer to status:

This mapper is "bad": it was used for something other than an actual cartridge. Some are mapper hacks designed for early disk-based copiers. Others are duplicate mappers that were assigned by mistake.

"Bad" mappers reserved for use by tools, such as emulator internal use or private use during development of a new mapper.

Mappers developed by members of the NES homebrew scene for cart releases.

"Pirate MMC3" mappers. Many are used for unauthorized demakes of 16-bit fighting games.

MMC1, MMC3: These mappers provide finely grained bank switching and features such as scanline counters. Their design involves an ASIC, which makes them more complicated and expensive than other mappers.